The invention relates to an optical amplifier arrangement having at least one optical amplifier stage and a variably settable attenuator.
Wavelength-division multiplexing (WDM) of optical transmission systems makes it possible to increase the transmission capacity of the optical transmission system, or WDM transmission system, by adding further WDM channels without any increase of the respective channel data rate. Optical amplifiers, or fiber amplifiers, arranged along the optical transmission path, are normally used to amplify simultaneously all the WDM channels, or WDM signals, transmitted via the optical transmission fiber. In order to avoid the problematic quality differences which occur in this case for the optical WDM channels at the end of the optical transmission path, it is necessary in particular that the WDM channels, or the WDM signals, have virtually the same signal level in each case at the amplifier inputs and outputs.
In optical WDM transmission systems, a variety of parameters which are responsible for, or give rise to, channel level differences in WDM signals are known. These include the wavelength dependency of the attenuation by passive components, which does not vary, or varies only slightly, during operation of the optical transmission path. Other effects are dependent on the operating state of the optical WDM transmission system, for example on the number of active WDM channels or on the ambient temperature, and hence result in variations during operation of the WDM transmission system.
In optical WDM transmission systems with large WDM channel numbers, which occupy a wide wavelength range and, at the same time, require a high total power at the input of the optical transmission fiber, a significant tilt of the channel level spectrum in each optical transmission path section is caused by the non-linear effect of stimulated Raman scattering (SRS) in the optical transmission fiber. In this case, the degree of tilt, or the level difference, of the respective WDM channel having the shortest and longest wavelength is dependent on the number of active WDM channels and is altered by switching further WDM channels on or off.
Furthermore, the channel level differences are produced, within the optical WDM signal to be transmitted, owing to the temperature dependency of the gain spectrum of the active fiber of optical fiber amplifiers. Especially in the case of erbium-doped fiber amplifiers, which are operated in the L transmission band (wavelength range of approximately 1570 to 1605 nm), any change in the ambient temperature leads to a significant tilt of the gain spectrum of the active fiber of the optical fiber amplifier.
Both SRS and the temperature dependency of the active fiber cause, at first approximation, a virtually dB-linear tilt of the channel level spectrum of the optical WDM signal. In the case of a dB-linear tilt, the level difference (in dB) between a WDM channel and its respectively neighbouring WDM channelxe2x80x94assuming a constant wavelength spacing of the WDM channelsxe2x80x94has the same value in each case for all WDM channels.
In WDM transmission systems produced to date, the wavelength dependency of the gain spectrum of the active fiber of optical fiber amplifiers is usually leveled by using an optical filter having inverse attenuation response, so as to reduce the channel level differences at the end of the optical transmission path which are due to the tilt of the gain spectrum. Such an optical filter is configured, for a given working point, in such a way that the remaining gain variation is limited to less than 1 dB over all the WDM channels. If the working point of the optical fiber amplifier is altered, i.e. if the gain of the active fiber is altered, then the gain spectrum tiltsxe2x80x94dynamic gain tilt.
One approach to compensating for the gain tilt involves splitting the optical amplifier into several optical amplifier stages. For instance, in the case of a 2-stage optical fiber amplifier, an attenuator which is preset to a fixed value and has a flat attenuation spectrum is interconnected between the first and second amplifier stages, and is used to compensate at least partially for the gain tilts in total due to the first and second amplifier stages. To that end, the individual amplifier stages are in each case regulated to a constant total output power, and the attenuator with a flat attenuation spectrum, which is interconnected between the amplifier stages, is therefore preset so that the gain of the two optical amplifier stages remains virtually constant.
It is furthermore possible to modify the regulation of the individual amplifier stages by regulating the individual amplifier stages not to a constant total output power, but rather to a constant gain. If the level at the input of the first amplifier stage rises, then its output level is also increased. The level at the input of the subsequent amplifier stage is kept constant since the attenuator with a flat attenuation spectrum, which is inserted between the amplifier stages, avoids passing on the level increase from its input to its output by increasing the attenuation value which is set. However, any increase in the attenuation between the amplifier stages impairs the noise properties of the overall two-stage amplifier arrangement.
When using a variable attenuator with a flat attenuation spectrum, only minor compensation for the tilt of the channel spectrum of the optical WDM signals, due to SRS or the temperature dependency of the active fiber of the optical fiber amplifier, is therefore possible.
A virtually dB-linear tilt of the channel level spectrum is also caused by a change in the gain of the active fiber of an optical fiber amplifier. The gain, or the gain spectrum, of the optical fiber amplifier can be adjusted, for constant input and output signal levels of the optical fiber amplifier, by changing the setting of a variable attenuator with a flat attenuation spectrum, so as to set a suitable tilt of the gain spectrum which virtually compensates for the constant part of the dB-linear tilt due to SRS or a temperature change. The disadvantage of this method is that it is only possible to reduce tilts, due to the active fiber of the optical fiber amplifier, which have a small amplitude. See, in particular, U.S. Pat. No. 5,530,583. To balance a strong tilt of, for example, 4 dB, it is therefore sometimes necessary to increase the inserted attenuation of the variable attenuator by more than 10 dB. Such a high additional inserted attenuation contributes to a significant increase in the noise factor of the optical fiber amplifier.
An advantage of the invention is to provide an optical amplifier arrangement with which it is possible to reduce channel level differences due to changes of the operating state of the optical WDM transmission system, for example as a result of switching channels on and off or temperature changes.
In an embodiment, on optical amplifier arrangement is provided having at least one optical amplifier stage (V1, V2) and a variably settable attenuator (VDL). The variably settable attenuator (VDL) has an attenuation spectrum (DV2, DV3, DV5, DV6) that increases or decreases proportionally to wavelength. The profile of the attenuation spectrum is variably settable in order to reduce any tilt of the channel level spectrum.
An aspect of the method according to the invention is that the variably settable attenuator has an attenuation spectrum that increases or decreases proportionally to wavelength, the profile of the attenuation spectrum being variably settable in order to reduce any tilt of the channel level spectrum. Advantageously, the channel level tilt of the optical transmission signal can be reduced with substantially less inserted attenuation, compared with compensation when using a settable attenuator with a flat attenuation spectrum, which leads to an improvement in the noise ratio of the optical amplifier arrangement. Furthermore, the attenuation spectrum with an inverse attenuation profile with respect to the tilt of the channel level spectrum can be approximated substantially more accurately by the variably settable attenuator than when using the dynamic gain tilt of the active fiber of the optical fiber amplifier.
A further advantageous aspect of the invention involves a variably settable attenuator that has a displaceably mounted filter, which is constructed from a transparent substrate into which ions or atoms with a suitable absorption spectrum are diffused, or onto which ions or atoms with a suitable absorption spectrum are deposited. In order to produce different attenuation values, the concentration of the absorbing ions or atoms increases or decreases along the displacement direction of the displaceably mounted filter. Such an embodiment of a variable settable attenuator, particularly intended to reduce tilts of the channel level spectrum within an optical amplifier arrangement, can be produced without any significant technical outlay and inexpensively. Furthermore, a wide variety of attenuation spectra can be produced for a plurality of possible tilts of the channel level spectrum by using the attenuator according to the invention. Such a variably settable attenuator can be provided at any desired points of an optical transmission system where, for example, dB-linear tilts of the channel level spectrum, for example of an optical transmission signal OS, are to be substantially compensated for or reduced.
In an embodiment, the optical amplifier arrangement, has an attenuation spectrum (DV2, DV3, DV5, DV6) that increases or decreases proportionally to wavelength is dB-linear.
In an embodiment, the optical amplifier arrangement, the increase of the dB-linear attenuation spectrum is set by the attenuation value (Dmax1, Dmax2) needed for the attenuation of the smallest or largest wavelength (xcex1, xcex2).
In an embodiment of the optical amplifier arrangement has a control unit or a regulating unit provided for driving the variably settable attenuator (VDL).
In an embodiment of the optical amplifier arrangement, a settable attenuator (VDF) with a flat attenuation spectrum, having virtually the same attenuation per channel, is provided for further reducing the dB-linear tilt of the channel level spectrum.
In an embodiment of the optical amplifier arrangement, an optical filter (GEF) is provided for leveling the gain spectrum of the at least one optical amplifier stage (V1, V2).
In an embodiment of the optical amplifier arrangement, a further variably settable attenuator is provided in addition to the one variably settable attenuator (VDL), the tilt of the channel level spectrum due to stimulated Raman scattering being reduced by the one variably settable attenuator and, separately, the further tilt of the channel level spectrum due to the temperature dependency of the active fibers of the at least one amplifier stage (V1) being reduced by the further variably settable attenuator.
In an embodiment of the optical amplifier arrangement, the one and the further variably settable attenuators (VDL) are arranged upstream or downstream of the at least one optical amplifier stage (V1, V2) or between at least two optical amplifier stages (V1, V2).
In an embodiment, a variably settable attenuator, in particular for reducing tilts of a channel level spectrum within an optical amplifier arrangement (OVA), has a displaceably mounted filter (F), which is constructed from a transparent substrate (TT) into which ions or atoms (AA) with a suitable absorption spectrum (a) are diffused, or onto which ions or atoms (AA) with a suitable absorption spectrum (a) are deposited. In order to produce different attenuation profiles (DV1 to DV5), the concentration of the absorbing ions or atoms (AA) increases or decreases along the displacement direction (A) of the displaceably mounted filter (F).
In an embodiment of the variably settable attenuator, a first and a second gradient lens (GL1, GL2) are provided, the displaceably mounted filter (F) being arranged between the first and second gradient lenses (GL1, GL2) and separated from them by a first and a second free-beam region (FB1, FB2).
In an embodiment of the variably settable attenuator, the first gradient lens (GL1) is intended to focus an optical signal (OS) injected via the input and to project the focused optical signal onto the displaceably mounted filter (F), and the second gradient lens (GL2) is intended to focus the optical signal (OS) attenuated by the displaceably mounted filter (F) and to project the focused, attenuated optical signal (OS) onto the output of the variably settable attenuator (VDL).
In an embodiment of the variably settable attenuator, the concentration of the absorbing ions or atoms (AA) increases or decreases at least virtually uniformly along the displacement direction (A) of the displaceably mounted filter.
In an embodiment of the variably settable attenuator, the displaceably mounted filter (F) is produced as a rotatably mounted filter disc (F) made of a transparent substrate (TT).
In an embodiment of the variably settable attenuator, the concentration of the absorbing ions or atoms (AA) is virtually constant at the site where the focused optical signal (OS) strikes the transparent substrate (TT).
In an embodiment of the variably settable attenuator, the variably settable attenuator (VDL) has a dB-linear attenuation profile (DV1 to DV6) that increases or decreases proportionally to wavelength (xcex).
In an embodiment of the variably settable attenuator, the attenuation properties of the variably settable attenuator (VDL) can be set by moving the displaceably mounted optical filter (F) along the displacement direction (A).
In an embodiment of the variably settable attenuator, the shape of the attenuation spectrum and of the usable wavelength range (WB) of the variably settable attenuator (VDL) is dependent on the diffused or deposited ions or atoms (AA1, AA2).
In an embodiment of the variably settable attenuator, a plurality of different ions and/or atoms (AA1, AA2) with different absorption spectra (S1, S2) are deposited onto the transparent substrate (TT) or are diffused into the transparent substrate (TT), their absorption spectra being superimposed.
In an embodiment of the variably settable attenuator, the focused optical signal (OS) is projected perpendicularly onto the transparent substrate (TT) of the displaceably mounted filter (F) by using the first gradient lens (GL1).
In an embodiment of the variably settable attenuator, the focused optical signal (OS) is projected, by using the first gradient lens (GL1), at an angle of less than 90 degrees onto the transparent substrate (TT), arranged along the displacement axis (A), of the displaceably mounted filter (F), in order to avoid reflections.
In an embodiment of the variably settable attenuator, the transparent substrate (TT) is divided into a first and a second attenuation region (DB1, DB2), whose absorption spectra (S1, S2) are different.
In an embodiment of the variably settable attenuator, the absorption spectra in each case have the smallest attenuation value (Dmin) on at least one contact line of the first and second attenuation regions (DB1, DB2), and the attenuation value (a) rises with increasing distance along the displacement axis (A) from the at least one contact line.
In an embodiment of the variably settable attenuator, the filter disc (F) is rotatably mounted about a rotation axis (A), and in that the concentration of the absorbing ions or atoms (AA) increases or decreases at least virtually uniformly along the rotation direction of the rotatably mounted filter disc (F).
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.